1. Field of the Invention
The present invention relates to a dielectric waveguide resonator and a dielectric waveguide filter for use particularly in a microwave or millimeter wave range, and to a method of adjusting the characteristics thereof.
2. Description of the Related Art
There are various types of dielectric resonators known for use in the microwave range. They include: a TE01xcex4-mode dielectric resonator consisting of a dielectric in the form of a solid circular cylinder or a hollow circular cylinder placed in a shield case; a TM110-mode dielectric resonator consisting of a prism-shaped dielectric which is placed in a metallic case or a case covered with a conducting film in such a manner that the dielectric extends from the upper to the lower faces of the case; and a TEM-mode dielectric resonator consisting of a dielectric wherein an inner conductor is disposed in the dielectric and the outer surface of the dielectric is covered with an outer conductor. These dielectric resonators have their own features and advantages and are used as microwave devices in various applications depending on particular purposes.
The size of these dielectric resonators can be reduced by confining the majority of resonating energy into a dielectric member and furthermore by forming a magnetic wall at a location close to a boundary plane between the dielectric member and air in such a manner that the magnetic wall is coincident with the even-mode symmetric plane. In these dielectric resonators, the resonance frequency and unloaded Q are determined by the size, shape, and dielectric constant of the dielectric resonator and the metallic case, and also by the location of the dielectric member in the metallic case.
In the case of a dielectric waveguide resonator consisting of a dielectric material such as a ceramic dielectric whose outer surface is covered with a conducting film, its size can be reduced by a factor of 1/{square root over (∈r+L )} relative to the size of a resonator in the form of a waveguide cavity where ∈r is the dielectric constant of the dielectric material. Thus, the dielectric waveguide resonator is expected to find applications in small-sized low-loss filters in the microwave and millimeter wave ranges. When a dielectric waveguide filter of such a type is combined with a microstrip line or a similar circuit element, the coupling between the dielectric waveguide filter and the external circuit is achieved by means of a structure such as those shown in FIGS. 33-35. In the example shown in FIG. 33, a conducting film 2 is formed on the outer surface of a dielectric block 1 so that the middle part of the dielectric block 1 serves as a waveguide system with a high Q, and coaxial TEM resonators are formed at either end of the dielectric block 1. In the example shown in FIG. 34, a conducting film 2 and stubs 9 are formed on the outer surface of a dielectric block wherein the coupling to the waveguide resonator system and the coupling to an external microstrip line are achieved via the stubs 9. In the example shown in FIG. 35, a hole is formed in a particular side of a dielectric block 1, and a probe 10 is inserted into the hole thereby achieving coupling to a waveguide resonance mode.
In the above-described conventional structures of dielectric resonators which operate in the TE01xcex4, TM110, or TEM mode, the resonance frequency and unloaded Q can be rather easily set to desired values by properly selecting the external dimensions. However, these dielectric resonators have problems in design and production arising from their structure. That is, in the TE01xcex4-mode dielectric resonator, a complicated structure is required to dispose a dielectric resonator at a particular fixed location in a shield case. In the case of the TM110-mode dielectric resonator, it is not easy to connect a prism-shaped dielectric to a metallic case or a case covered with a conducting film through which a current flows. When the prism-shaped dielectric and the outer conductor are combined in an integral fashion, a complicated and difficult molding technique is required. Furthermore, it is required that an end of the case be open so as to process the prism-shaped dielectric in the case. When the resonator is used, it is required to cover the open end with a conductor. This causes an increase in the cost of the production and assembly process. On the other hand, in the case of a TEM-mode dielectric resonator, the outside dimensions should be great enough to obtain a high unloaded Q. However, if the outside dimensions are increased, the resonance frequency in a high-order resonance mode becomes close to the primary resonance frequency in the TEM mode to be used. Since only a certain number of dielectric materials are available in practical production, the unloaded Q is limited within a certain range. In the case where a band-pass filter is constructed of a dielectric block having a plurality of inner conductor holes and having a coupling hole formed in the middle of each inner conductor hole wherein the coupling between resonators is adjusted by properly selecting the effective dielectric constant between resonators, it is required that only the inner surface of each inner conductor hole be covered with an inner conductor while the inner surface of the coupling holes should remain uncovered. However, this requires a complicated production process.
It is also known in the art to construct a dielectric waveguide resonator by forming a conducting film on the outer surface of a ceramic dielectric. This structure is equivalent to a cavity resonator filled with a dielectric. If a dielectric with a dielectric constant of ∈r is employed, a reduction in wavelength occurs and thus it is possible to reduce the total size of the resonator by a factor equal to 1/{square root over (∈r+L )}. FIG. 31 illustrates the structure of a TE101-mode dielectric waveguide resonator. The wavelength inside the resonator is given by xcexg=2ac/{square root over (a2+L +c2+L )}, and this wavelength determines the resonance frequency. The unloaded Q is determined by the wavelength xcexg, the skin depth xcex4 of the conducting film formed on the surface of the dielectric, and the dimensions a, b, and c of the dielectric block wherein the unloaded Q increases with the dimensions a, b, and c. Although this type of dielectric waveguide resonator requires a greater size for the same resonance frequency than a coaxial dielectric resonator, it is easy to produce a resonator having a high unloaded Q. However, in this type of dielectric waveguide resonator, when the dielectric constant ∈r of the ceramic dielectric used and the main resonance frequency as well as adjacent resonance frequency are given, the dimensions a, b, and c of the resonator are determined by the given parameters, and the unloaded Q is determined by the dimensions a, b, and c. This requires the dielectric constant ∈r of the dielectric material to be within the range around 20, from 30 to 35, or around 90. In practice, it is difficult to freely select the dielectric constant. Therefore, when a desired resonance frequency is achieved using a given dielectric material, the only parameter allowed to vary to adjust the unloaded Q is the dimension b. In this case, it is required to properly select the dimension b while taking into account the effect of the adjacent resonance frequency on the main resonance frequency. Thus, this type of resonator is difficult to design and adjust.
In view of the above, it is an object of the present invention to provide a dielectric waveguide resonator whose resonance frequency and unloaded Q can be designed in a more flexible fashion, and can be easily adjusted to desired values.
FIG. 33 illustrates the structure of a conventional dielectric waveguide filter. Although this type of dielectric waveguide filter can be easily coupled to a microstrip line, the coaxial resonator portions have a low unloaded Q relative to that of the waveguide resonator, and thus the overall unloaded Q becomes low. On the other hand, in the case of the stricture shown in FIG. 34, it is required that the length of the stub 9 should be large enough to achieve strong coupling. However, the long stub 9 can cause leakage of electromagnetic waves via the gap between the stub 9 and the conducting film 2. The leakage of electromagnetic wave can cause interference in an external circuit. In the structure shown in FIG. 35, it is required that a probe 10 should be prepared separately from the resonator. Furthermore, it is also required to securely fix the probe 10 relative to the dielectric block 1.
Thus, it is another object of the present invention to provide a dielectric waveguide resonator having a simple coupling circuit element by which coupling to an external circuit can be achieved without having to use an additional special member and without causing a great amount of leakage of electromagnetic waves toward the outside.
According to an aspect of the present invention, there is provided a dielectric waveguide resonator including a dielectric block whose outer surface is covered with a conducting film, the dielectric waveguide resonator being characterized in that a through-hole whose inner surface is not covered with a conducting film is formed in the dielectric block in such a manner that the through-hole extends from one face to another face of the dielectric block or a recess whose inner surface is not covered with a conducing film is formed on a particular face of the dielectric block thereby adjusting the resonance frequency and the unloaded Q. As a result of the formation of the through-hole or recess whose inner surface is not covered with a conducting film in the dielectric block, the dielectric constant in the through-hole or recess becomes different from that of the dielectric block and resultant perturbation effect on the electric field causes an increase in the resonance frequency. Therefore, this technique makes it possible to adjust the resonance frequency by properly selecting the size and/or location of the through-hole or recess while keeping the outside dimensions of the dielectric block constant. Thus, it is possible to set the resonance frequency and unloaded Q to desired values over wide ranges by properly designing the outside dimensions of the dielectric block and the size or location of the through-hole or recess. This makes it possible to design the unloaded Q in a more flexible fashion.
In the case of a dielectric waveguide resonator consisting of a rectangular dielectric block whose outer surface is covered with a conducting film, such as that shown in FIGS. 32A-32B, if the dielectric block has a dielectric constant ∈r of 21, and the size thereof is given by a=23 mm, b=9 mm, and c=18 mm, then the resonance frequency fo becomes about 2.5 GHz. Although it is also possible to adjust the resonance frequency fo by removing a particular portion over an area of for example 2 mm square from the conducting film on a side face of the dielectric block as shown in FIG. 32B, a change in the resonance frequency fo as great as about 1000 ppm will occur when a metallic element is placed near the above removed portion of the conducting film. Such a great change of 1000 ppm in fo will result in a great change in the characteristics of the multi-stage filter. In contrast, in the case where a through-hole whose inner surface is not covered with a conducting film is formed in a dielectric block as shown in FIG. 32A, only a small change of about 100 ppm occurs in fo when a metallic element is placed near the open plane of the through-hole. Furthermore, in the case of the structure shown in FIG. 32B in which the conducting film on a side face is partially removed, about a 10% reduction occurs in the unloaded Q. In contrast, substantially no change in the unloaded Q occurs in the case of the structure shown in FIG. 32A in which the through-hole whose inner surface is not covered with a conducting film is formed in the dielectric block.
According to another aspect of the present invention, the through-hole or recess is formed at a location at which the electric field distribution has a high electric strength in a particular resonance mode. This makes it possible to produce a relatively great change in the resonance frequency by forming a small through-hole or recess. This technique also makes it possible to design the unloaded Q within an expanded range.
When a resonator can have a plurality of resonance modes, it is possible to construct a plurality of dielectric resonators with a single dielectric block by utilizing the individual resonance modes, and it is also possible to combine these resonance modes to realize a filter. For example, when the dielectric resonator has first and second resonance modes, if the through-hole or recess is formed at a location at which the electric field strength in the second resonance mode is greater than that in the first resonance mode, it is possible to adjust selectively only the resonance frequency in the second resonance mode relative to the resonance frequency in the first resonance mode even in the case where the resonance frequencies in the first and second resonance modes are close to each other. Thus, this technique makes it easy to adjust the difference in resonance frequency between two resonance modes. According to another aspect of the present invention, the through-hole or recess is preferably formed at a location at which the electric field strength in the first resonance mode is nearly equal to that in the second resonance mode. In this case, the resonance frequencies in the first and second resonance modes are equally affected by the through-hole or recess and thus it is possible to simultaneously set the resonance frequencies in the two resonance modes to desired values simply by adjusting the single through-hole or recess.
In still another aspect of the present invention, the two resonance modes may be degenerated by forming the dielectric block into a rectangular block shape in which at least two opposite side faces are squares, or into a solid circular cylinder or hollow circular cylinder.
If the through-hole or recess is formed in a direction along the electric field in a particular resonance mode, it is possible to enhance the perturbation effect on the electric field. Furthermore, if the through-hole or recess is formed into a tapered shape or a stepped shape, it becomes easy to make coarse and fine adjustments on the resonance frequency by properly forming the through-hole or recess.
Although the through-hole or recess may be hollow (that is filled with air), a dielectric material having a dielectric constant different from that of the dielectric block may also be placed in the through-hole or recess.
In a further aspect of the invention, the opening end of the through-hole or recess is covered with a conductor thereby ensuring that leakage of electromagnetic waves toward the outside or unwanted electromagnetic coupling with an external circuit is prevented.
According to another aspect of the present invention, there is provided a dielectric waveguide filter including a dielectric block whose outer surface is covered with a conducting film, the dielectric waveguide filter being characterized in that a terminal electrode isolated from the conducting film is formed on the outer surface of the dielectric block and a hole is formed in the dielectric block wherein a coupling electrode is formed on the inner surface of the hole in such a manner that one end of the coupling electrode is connected to the terminal electrode and the other end of the coupling electrode is connected to the conducting film. This makes it possible to reduce the leakage of electromagnetic waves toward the outside without having to use an additional special member. In this structure a coupling loop is formed by the coupling electrode and the conducing film disposed on the outer surface of the dielectric block, thereby providing magnetic coupling to a resonance mode of the dielectric waveguide resonator occurs.
According to another aspect of the invention, a terminal electrode isolated from the conducting film is formed on the outer surface of the dielectric block and a hole is formed in the dielectric block wherein a coupling electrode is formed on the inner surface of the hole in such a manner that one end of the coupling electrode is connected to the terminal electrode and the other end of the coupling electrode is electrically open-circuited in the hole. In this structure, the coupling electrode serves to provide coupling to the electric field in a resonance mode of the dielectric waveguide resonator. In any of these structures described above, a connection to an external circuit element such as a microstrip line can be made via the terminal electrode formed on the outer surface of the dielectric block wherein the terminal electrode is connected to one end of the coupling electrode. The above connection can be achieved without having to insert an additional special member such as a probe into the hole from the outside. Furthermore, this structure provides excellent coupling to the external circuit element without producing leakage of electromagnetic waves toward the outside.
According to still another aspect of the invention, the above-described hole includes a hole extending in a substantially straight line and a hole intersecting the former hole. This makes it possible to form a coupling electrode in a flexible fashion in the dielectric block.
According to another aspect of the invention, a hole whose inner surface is not covered with a conducting film is formed in the dielectric block and a pin-shaped conductor covered with an insulating material is inserted in the above hole so that coupling with an external circuit is achieved via the pin-shaped conductor. Thus, this technique allows a simplification of the structure of the dielectric block and also allows easier coupling to the external circuit.
According to still another aspect of the invention, a slot whose inner surface is covered with a conducting film is formed in the dielectric block so that the slot acts as a node by which the dielectric block is divided along the direction of its length. This technique makes it possible to realize a multi-stage dielectric waveguide filter with a single dielectric block.
According to another aspect of the invention, there is provided a method of adjusting the characteristics of dielectric waveguide filter, including the step of partially removing the coupling electrode, which is formed on the inner surface of the hole, thereby adjusting the amount of coupling to an external circuit. In this method, it is possible to easily adjust the amount of coupling to the external circuit simply by partially removing the coupling electrode without having to use an additional special adjustment member and without producing leakage of electromagnetic waves toward the outside.
According to still another aspect of the invention, there is provided a method of adjusting the characteristics of dielectric waveguide filter, including the step of partially removing the inner surface of the through-hole or recess.
Other features and advantages of the invention will be understood from the following detailed description of embodiments thereof and the accompanying drawings, in which like references illustrate like elements and parts.